Deep eutectic solvent-mediated green synthesis of oxygen-deficient 2D HfO_2-x nanosheets as a fluorescent probe for...

Addressing the critical need for sustainable environmental monitoring, we report the development of a selective fluorescent sensor for hexavalent chromium (Cr(VI)) using a biodegradable deep eutectic solvent (DES) as both a reaction medium and a structure-directing agent. Two-dimensional oxygen-deficient hafnium oxide nanosheets (γ-HfO_2-x) were fabricated via a one-step, additive-free hydrothermal process, eliminating the need for toxic organic solvents. The resulting γ-HfO_2-x probe exhibits distinct green fluorescence (458 nm) that is selectively quenched in the presence of Cr(VI). Comprehensive mechanistic investigations, including time-resolved photoluminescence, confirm that the quenching is governed by the inner filter effect (IFE). The sensor achieves a detection limit of 0.233 μM, good selectivity against competing metal ions, and reliable performance in real tap and river water matrices. Comparative analysis with systems synthesized in deionized water and an ethylene glycol-based DES highlights the morphological and optical advantages associated with the glycerol-DES. This work demonstrates the potential of DES-based green synthesis for developing functional nanomaterials, offering an eco-friendly platform for on-site water quality monitoring.